If you want to have the main archive file with a PHP extension and not as sub-folder, make the following changes to functions.php file:
FIND:
Code:
$bburl . '/archive/
REPLACE WITH:
Code:
$bburl . '/archive.php
NOTE: The code appears 2 times, change it in both places.
Rename the "archive" to archive.php.
I personally don't recommend you to do this, instead use both ways...
Leave the script unchanged but upload also archive.txt and rename it to archive.php, the crawlers will hit both files.